This application is based on Japanese patent application Nos. H10-198798 and H10-198799, both filed on Jul. 14, 1998, the entire contents of which are incorporated herein by reference.
The invention relates to a polyisocyanate compound and a process for producing the same. In particular, it relates to a novel polyisocyanate compound useful as a starting material for an optical material, and a process for producing the same at good efficiency. It also relates to an optical material and an optical product using the same. Specifically, it relates to an optical material which has excellent optical characteristics such as a high refractive index, a low dispersion, an excellent transparency, lack of optical distortion and the like and good heat resistance, and to an optical product formed of this optical material, such as a lens, a prism, optical fibers, a substrate for a recording medium, a filter, a glass, a vase and the like.
Recently, plastic materials have been used for optical parts in place of inorganic glass, especially for eyeglass lenses, because they are lightweight and have good dyeability and good impact resistance. In particular, a polyethylene glycol bisallylcarbonate polymer (hereinafter referred to as CR-39) and polymethyl methacrylate (PMMA) have been used, primarily for plastic lenses. However, it has a refractive index of 1.50, which is lower than that of inorganic glass. Therefore, when they are used as lens materials, for example, the thickness of the lenses has to be increased with the increasing power. Consequently, not only is the superiority of plastics as lightweight material been impaired, but also they have not been preferred in view of the aesthetic property. Furthermore, the concave lens, in particular, has had problems when the thickness (edge thickness) of the surrounding portion of the lens is increased causing a birefringence or a chromatic aberration to occur.
For these reasons, in order to be able to decrease the thickness of the lens while applying most of the characteristics of plastics having a low specific gravity, a plastic material having a high refractive index has been in demand. As a material having such a performance, for example, (1) a polymer comprising a xylene diisocyanate compound and a polythiol compound (official gazette of JP-A-63-46213), (2) a resin comprising an aliphatic linear sulfur-containing diisocyanate and a polythiol compound (official gazette of JP-A-2-153302), (3) a polymer comprising a dithian derivative substituted with two isocyanatoalkyl groups and a polythiol (official gazette of JP-A4-159275) are disclosed.
Although the resin in the above-mentioned item (1) and the resin in the above-mentioned item (2) have an increased refractive index by limiting a combination with a polythiol compound to be polymerized, the resin of item (1) has problems in that an Abbe number is decreased and a chromatic aberration is increased, and the resin of item (2) has a problem in that its heat resistance is decreased.
The polymer of item (3) has a high refractive index and a low dispersion (high Abbe number) and has an excellent heat resistance. However, in recent years, there has been a demand for a material having a higher refractive index and a lower dispersion.
In order to overcome many of the problems in the prior art, the invention provides an optical material having the above-mentioned desirable properties. It is a feature of the invention to provide a novel polyisocyanate compound which can give an optical material having a high refractive index, a low dispersion and an excellent heat resistance, and a process for producing this compound at good efficiency. It is also a feature of the invention to provide an optical material which has excellent optical characteristics such as a high refractive index, a low dispersion, an excellent transparency, lack of optical distortion and the like, and which has good heat resistance. It is also a feature of the invention to provide optical products made from the optical material.
In particular, the invention provides a polyisocyanate compound having a 1,4-dithian ring and an isocyanate group in a side chain connected by a sulfur atom. The compound exhibits a high refractive index, a low dispersion and high heat resistance. In addition, the invention provides methods for producing the polyisocyanate compound by an efficient processes. It also provides an optical material formed of a poly(thio)urethane obtained by subjecting a component comprising a polyisocyanate compound and a component comprising a compound having two or more of hydroxyl groups or mercapto or both groups to a polyaddition reaction.
In one aspect, the invention provides a polyisocyanate compound having general formula (I) 
wherein X and Y independently represent an alkylene group having from 1 to 5 carbon atoms or an alkylene group having from 1 to 5 carbon atoms to which group an isocyanatoalkyl group is bound, wherein the carbon number of the alkyl group of the isocyanatoalkyl group is between 1 and 3 the alkylene groups optionally contain a sulfur atom, and m and n independently represent an integer from 1 to 3.
In another embodiment, the polyisocyanate compound can be produced by one of the following processes: production process 1 and production process 2.
Production process 1 of the invention is a process for producing a polyisocyanate compound represented by general formula (I-a) 
wherein m and n, independently represent an integer from 1 to 3, X1 and Y1, independently represent an alkylene group having from 1 to 5 carbon atoms, and this alkylene group may have a sulfur atom.
Process 1 comprises:
(a) obtaining a dicarboxylic acid ester from a 2,5-bis(mercaptoalkyl)-1,4-dithian compound, said dicarboxylic acid being represented by general formula (III) 
wherein R1 and R2, independently represent a lower alkyl group, X1, Y1, m and n are as defined above, said 2,5-bis(mercaptoalkyl)-1,4-dithian compound being represented by general formula (II) 
wherein m and n are as defined above,
(b) converting the dicarboxylic acid ester to a dicarbonyl hydrazide represented by general formula (IV) 
wherein m, n, X1 and Y1 are as defined above,
and
(c) converting the carbonyl hydrazide group into an isocyanate group.
Production process 2 of the invention is a process for producing a polyisocyanate compound represented by general formula (I-b) 
wherein A1 and A2, independently represent a substituted alkylene group having from 1 to 5 carbon atoms, this alkylene group may have a sulfur atom, B1 and B2, independently represent an alkylene group having from 1 to 3 carbon atoms, m and n, independently represent an integer from 1 to 3, and k represents 0 or 1.
Process 2 comprises:
(a) obtaining a tricarboxylic acid ester or a tetracarboxylic acid ester from a 2,5-bis(mercaptoalkyl)-1,4-dithian compound, the tricarboxylic acid ester or the tetracarboxylic acid ester being represented by general formula (V) 
wherein A1, A2, B1, B2, k, m and n are as defined above, and R3 and R4, independently represent a lower alkyl group, the 2,5-bis(mercaptoalkyl)-1,4-dithian compound being represented by the above-mentioned general formula (II),
(b) converting the tricarboxylic acid ester or a tetracarboxylic acid ester to a tricarbonyl hydrazide or a tetracarbonyl hydrazide represented by general formula (VI) 
wherein A1, A2, B1, B2, m, n and k are as defined above, and
(c) converting the carbonyl hydrazide group into an isocyanate group.
In another embodiment, the optical material of the invention is formed of a poly(thio)urethane comprising a polyisocyanate compound represented by general formula (I): 
The polyisocyanate compound has an alicyclic structure. Preferably, the compound comprises isocyanatoalkyl groups having at least one sulfur atom and the groups are bound to the 2- and 5-positions of the 1,4-dithian ring.
In another aspect of the invention, in the above-mentioned general formula (I), X and Y, independently represent an alkylene group having from 1 to 5 carbon atoms or an alkylene group having from 1 to 5 carbon atoms to which group an isocyanatoalkyl group (in which the carbon number of the alkyl group is preferably between 1 to 3) is bound, and these alkylene groups may have a sulfur atom. It is preferable that the sulfur atoms are bound to two carbon atoms in the alkylene groups. The X and Y groups are independently a methylene group or an ethylene group, or a group represented by the formula: 
The groups are preferably: (a) a methylene group to which an isocyanatomethyl group is bound, (b) an ethylene group in which an isocyanatomethyl group is bound to a carbon atom bound to a sulfur atom or (c) an ethylene group in which an isocyanatomethyl group is bound to a carbon atom bound to an isocyanate group.
It is also preferred that m and n, independently represent an integer from 1 to 3.
It is preferred that the polyisocyanate compound represented by general formula (I) contains not only the 1,4-dithian ring but an additional sulfur atom outside the 1,4-dithian ring, preferably positioned to increase the refractive index and the Abbe number of the polyisocyanate compound. Accordingly, when an optical material is produced using this polyisocyanate compound, the refractive index and the Abbe number of the optical material are also increased.
Furthermore, it is preferred that the 1,4-dithian ring in the polyisocyanate compound is rigid. Therefore, when an optical material is produced using this, excellent mechanical characteristics can be given to the optical material.
Furthermore, it is preferred that in the polyisocyanate compound, when the isocyanatoalkyl groups (in which the carbon number of the alkyl group is preferably between 1 and 3) are bound to the alkylene groups having from 1 to 5 carbon atoms which groups may have a sulfur atom as indicated at X and Y in general formula (I), this compound itself becomes a crosslinking agent.
When an optical material is produced using the polyisocyanate compound, high heat resistance and a high solvent resistance can be imparted to the optical material without adding the other crosslinking agent as a secondary component.